I've been carefully studying the DC to DC converter article in March 2009 N&V. I am confused on one point. On page 48, u (mu) for the toroid made of 3C90 material is given, "From the 3C90 specification sheet," as 4 x 10 to the minus 3. Yet as far as I can see on the spec sheet from the manufacturer, Ferroxcube, the value of u is listed as being between 2300 and 5500.